While working through Theogony, I came across a usage of the dative case I haven't seen before (I'm best at Biblical Greek, but am not unfamiliar with Homer and have translated hundreds of lines of the Iliad and Odyssey). I suppose one could call it a "dative of comparison," but I've looked in Smyth under his chapter on the dative, as well as in Monro's Homeric Grammar and Wallace's Greek Grammar Beyond the Basics and I can't find this sort of usage noted.

What is going on here with ἐτύμοισιν here? Sure, it's an adjective being used substantively as a noun in the dative case, but what syntactic category does this dative fall under? In what section of Smyth (or other grammar) is this discussed?

Here is line 27 from Hesiod's Theogony:

ἴδμεν ψεύδεα πολλὰ λέγειν ἐτύμοισιν ὁμοῖα,

which I translate as "we know how to say many false things like true things," probably meaning something like "we know how to say many false things as if they were true things."

This is extremely similar to Odyssey 19.203:

ἴσκε ψεύδεα πολλὰ λέγων ἐτύμοισιν ὁμοῖα.

Any input or discussion of this usage of the dative would be appreciated.

Ah, thanks. Now I see where to find it in the usual grammars. Wallace (p. 174) notes “A few adjectives are followed by the dative case…the most common group is adjectives of “likeness” such as ὅμοιος, ἴσος.” Smyth (§1501) notes examples of the dative used with adjectives and adverbs of similarity and dissimilarity, noting its use with ὅμοιος, of course, but also with ἐναντίος. Pharr gives additional examples with other adjectives in §1003.